Method for forming film on aluminum or aluminum alloy, pretreatment liquid therefor, and product thereof

ABSTRACT

In a method for forming a film on a surface of aluminum or an alloy thereof, first, an anodic oxide film is formed on a surface of aluminum, a pretreatment is performed by immersing the aluminum or the alloy thereof on which the anodic oxide film is formed in a pretreatment liquid, and a sealing treatment using a sealing treatment liquid containing lithium ions is performed on the anodic oxide film on the surface of the aluminum or the alloy thereof subjected to the pretreatment. The pretreatment liquid contains phosphate ions and a reaction-controlling agent and has a pH in a neutral to acidic range. The reaction-controlling agent contains a compound having a carboxy group or a salt thereof or contains a compound capable of forming a hydroxide ion in an aqueous solution. This method makes it possible to uniformly form a sealing product in pores of the film.

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority from Japanese Patent Application No.2014-115918 filed Jun. 4, 2014, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a film on aluminumor an aluminum alloy, a pretreatment liquid therefor, and a productthereof. More specifically, the present invention relates to a sealingtreatment for closing pores present in an anodic oxide film formed on asurface of aluminum or an aluminum alloy.

2. Description of the Related Art

The anodic oxidation treatment is employed as a method for improving thecorrosion resistance of aluminum or an aluminum alloy. By the anodicoxidation treatment, an anodic oxide film is formed on a surface of thealuminum or the aluminum alloy. The anodic oxide film has a porous layerin which a large number of fine pores are present, and the porous layeris one of the causes of decrease in corrosion resistance. For thisreason, to further improve the corrosion resistance, a sealing treatmentfor closing the fine pores is carried out after the anodic oxidationtreatment.

A method employed for the sealing treatment is a high-temperaturehydration type sealing treatment method in which an aluminum workpiecehaving an anodic oxide film formed thereon is immersed in boiling water.However, this method has problems such as the treatment time must be 10minutes or more and a large amount of energy is necessary to maintainthe boiling state of the boiling water. To solve these problems, amethod using an aqueous solution containing lithium ions as a sealingtreatment liquid has been developed, as disclosed in Japanese PatentApplication Publication No. 2010-077532. In this sealing treatmentmethod, the sealing treatment can be carried out even with thetemperature of the treatment liquid being 25° C. and the treatment timebeing 1 minute. Hence, this sealing treatment method is excellent interms of the production efficiency and energy consumption.

In addition, Japanese Patent Application Publication No. 2013-1957discloses a method for performing a sealing treatment on a surface of ananodic oxide film formed on a surface of an aluminum workpiece. Asealing treatment liquid used in this method contains lithium ions andtwo reaction accelerators, that is, a reaction accelerator A selectedfrom polyoxyethylene alkyl ethers and the like and a reactionaccelerator B selected from polyvinyl alcohol and the like. In addition,the following reaction conditions are described: the sealing treatmentliquid is made weakly alkaline with a pH of 8.0 to 10.0; and theimmersion time in the sealing treatment liquid is 30 minutes.

SUMMARY OF THE INVENTION

The sealing treatment method using lithium ions disclosed in JapanesePatent Application Publication No. 2010-077532 is a method in which thesealing is carried out rapidly by causing the anodic oxide film topartially dissolve and form a compound with lithium ions. For thisreason, the amount of the sealing product formed in the pores variesbetween the pore bottom portion and the surface portion, and a largeramount of the sealing product is formed near the surface of the anodicoxide film. As a result, the color tone of the anodic oxide film turnscloudy white after the sealing. This results in a problem in that theappearance quality deteriorates in the case in which a colorless paintor no paint is applied onto the anodic oxide film.

The sealing treatment method disclosed in Japanese Patent ApplicationPublication No. 2013-1957 makes it possible to prevent the color tone ofthe anodic oxide film from turning cloudy white; however, this sealingtreatment method has the following problem. Specifically, even when theworkpiece is washed with water after the anodic oxidation treatment, theanodic oxidation treatment liquid remains on the workpiece, because thesealing treatment liquid is weakly alkaline with a pH of 8.0 to 10.0.For this reason, the contamination of the sealing treatment liquid withthe anodic treatment liquid, which is strongly acidic or strongly basic,is unavoidable in the next step. Accordingly, the pH of the sealingtreatment liquid is changed greatly, and it is difficult to maintain thepH within a predetermined pH range. It is described that when the pH ofthe sealing treatment liquid is 12.0, the dissolution of the film occursat a surface portion of the anodic oxide film, and the finished surfacehas a whitish and uneven color.

In this respect, an object of the present invention is to provide amethod for forming a film on aluminum or an aluminum alloy which makesit possible to prevent change in color tone of a surface of an anodicoxide film after a sealing treatment, reduce the time taken for thesealing treatment, and moreover, easily control the pH of the sealingtreatment liquid, and also to provide a pretreatment liquid therefor anda product thereof.

To achieve the above-described object, an aspect of the presentinvention is a method for forming a film on a surface of aluminum or analuminum alloy, the method including the steps of: forming an anodicoxide film on a surface of aluminum or an aluminum alloy; performing apretreatment by immersing, in a pretreatment liquid, the aluminum or thealuminum alloy on which the anodic oxide film is formed; and performinga sealing treatment using a sealing treatment liquid containing lithiumions on the anodic oxide film present on the surface of the aluminum orthe aluminum alloy subjected to the pretreatment, wherein thepretreatment liquid contains phosphate ions and a reaction-controllingagent and has a pH in a neutral to acidic range, and thereaction-controlling agent is a compound having a carboxy group or asalt thereof or is a compound capable of forming a hydroxide ion in anaqueous solution.

A concentration of the phosphate ions is preferably 0.1 to 4.0 mol/L. Aconcentration of the reaction-controlling agent is preferably at least0.05 mol/L. A temperature of the pretreatment liquid is preferably 10 to50° C. An immersion time in the pretreatment liquid is preferably atleast 1.5 minutes. The compound having a carboxy group is preferably acarboxylic acid or a salt thereof. The compound capable of forming ahydroxide ion in an aqueous solution is preferably a hydroxide. Theanodic oxide film may have a thickness of 3 to 40 μm. In the sealingtreatment step, a sealing product is formed in pores present in asurface of the anodic oxide film, and the sealing product may be alithium-aluminum hydrate containing at least LiH(AlO₂)₂.5H₂O. Thepretreatment liquid is preferably a buffer solution.

Another aspect of the present invention is a pretreatment liquid usedfor a pretreatment in a sealing treatment on an anodic oxide film byusing a sealing treatment liquid containing lithium ions, thepretreatment liquid including: phosphate ions; and areaction-controlling agent, wherein the pretreatment liquid has a pH ina neutral to acidic range, and the reaction-controlling agent contains acompound having a carboxy group or a salt thereof or contains a compoundcapable of forming a hydroxide ion in an aqueous solution.

A still another aspect of the present invention is an aluminum oraluminum alloy product including: aluminum or an aluminum alloy; and ananodic oxide film formed on a surface of the aluminum or the aluminumalloy, wherein pores in a surface of the anodic oxide film formed on thealuminum or the aluminum alloy are sealed with a sealing product, thesealing product contains a lithium-aluminum hydrate, and thelithium-aluminum hydrate is uniformly formed from the surface of theanodic oxide film to pore bottom portions in the pores.

As described above, according to the present invention, the pretreatmentstep is performed before the sealing treatment step, and phosphate ionsand a reaction-controlling agent are added to the pretreatment liquidused in this pretreatment step. In addition, the pH of the pretreatmentliquid is adjusted to the neutral to acidic range. Moreover, a compoundhaving a carboxy group or a salt thereof or a compound capable offorming a hydroxide ion in an aqueous solution is used as areaction-controlling agent. Hence, phosphate ions are moderatelycaptured by the anodic oxide film in the pretreatment step. Thephosphate ions have an effect of inhibiting the formation of sealingproducts such as a lithium-aluminum hydrate formed in the sealingtreatment step. Hence, it is possible to form the sealing productuniformly from the film surface to the pore bottom portions in multiplefine pores present in the surface of the anodic oxide film. This makesit possible to prevent the change in color tone of the anodic oxidefilm. In addition, each of the pretreatment step and the sealingtreatment step is completed in a short period of about several minutes,and the pH of the pretreatment liquid is in the neutral to acidic range.Hence, the pH of the sealing treatment liquid can be controlled easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating an anodicoxide film subjected to a sealing treatment and obtained by a filmformation method according to the present invention.

FIG. 2 is a cross-sectional view schematically illustrating an anodicoxide film subjected to a sealing treatment and obtained by a referencefilm formation method.

FIG. 3 shows photographs of the appearance of surfaces of anodic oxidefilms subjected to sealing treatments in the Examples and ReferenceExamples.

FIG. 4 is an electron micrograph of a surface of an anodic oxide filmsubjected to a sealing treatment in the Examples.

FIG. 5 shows photographs of the appearance of surfaces of anodic oxidefilms subjected to a sealing treatment in the Examples and ComparativeExamples after a corrosion resistance test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of a film formation method according to thepresent invention is described. The film formation method of thisembodiment includes the steps of: forming an anodic oxide film on asurface of aluminum or an aluminum alloy; performing a pretreatment byimmersing, in a pretreatment liquid, the aluminum or the aluminum alloyon which the anodic oxide film is formed; and performing a sealingtreatment using a sealing treatment liquid on the anodic oxide filmpresent on the surface of the aluminum or the aluminum alloy subjectedto the pretreatment.

The material on which the film is to be formed may be aluminum or mayalso be an aluminum alloy. The aluminum alloy can contain a wide varietyof components such as silicon and copper, without any particularlimitation. A method for producing the aluminum or the aluminum alloy isnot particularly limited, and this film formation method can be appliedalso to wrought materials, cast materials, and die-cast materials.

In the step of forming an anodic oxide film, an anodic treatment liquidis electrolyzed by using the aluminum or the aluminum alloy as a workingelectrode in the anodic treatment. Thus, an anodic oxide film mainlycontaining aluminum oxide can be formed on a surface of the aluminum orthe aluminum alloy. The anodic treatment liquid is not particularlylimited, and, for example, either an acidic bath of sulfuric acid,oxalic acid, phosphoric acid, chromic acid, or the like, or an alkalinebath of sodium hydroxide, sodium phosphate, sodium fluoride, or the likemay be used.

A method for the electrolysis is not particularly limited, and any oneof direct-current electrolysis, alternating-current electrolysis, AC-DCsuperimposition electrolysis, Duty electrolysis, and the like may beused. In addition, it is preferable to perform washing with water atleast once after the electrolytic treatment. By performing the washingwith water, the anodic treatment liquid attached to the aluminum or thealuminum alloy can be removed to some degree, or the concentration ofthe treatment liquid which cannot be removed can be reduced. Thus, theamount of the treatment liquid carried over to the next step can bereduced. Water used for the washing is preferably water with a lowerlevel of impurities, such as ion-exchanged water or purified water.

The film thickness of the anodic oxide film is not particularly limited,and is preferably 3 to 40 μm. Since a sealing treatment using lithiumions is performed on the anodic oxide film, the anodic oxide film has tohave a certain thickness. When the anodic oxide film has a thickness of3 μm or more, the anodic oxide film is not lost even when subjected tothe sealing treatment using lithium ions. Moreover, when the anodicoxide film has a thickness of at least 40 μm, the anodic oxide film canexhibit functions sufficient for a film of aluminum or an aluminumalloy. In addition, it is possible to improve productivity because thetime necessary for the film formation can be reduced.

In the pretreatment step, the aluminum or the aluminum alloy on whichthe anodic oxide film is formed is immersed in a pretreatment liquidcontaining phosphate ions and a reaction-controlling agent and having apH in an acidic to neutral range, before a sealing treatment on theanodic oxide film.

By immersing the anodic oxide film in such a pretreatment liquid,phosphate ions are captured on the surface and in pores of the anodicoxide film. Since phosphate ions have a function of inhibiting ahydration reaction, the formation reaction of a lithium-aluminumhydrate, which is a sealing product requiring a large number of watermolecules, proceeds gently, so that the variation between the amounts ofthe sealing product formed in pore bottom portions in the pores andformed in a surface portion can be reduced. For this reason, it ispossible to reduce change in color tone of the anodic oxide film due toreflection or refraction of light falling on the film.

As a phosphate ion source, phosphoric acids and phosphoric acid salts,which are water-soluble and contain phosphate ions, are preferable.Examples of the phosphate ion source include orthophosphoric acid,disodium hydrogen phosphate, sodium dihydrogen phosphate, trisodiumphosphate, potassium phosphate, calcium dihydrogen phosphate, lithiumphosphate, ammonium phosphate, and the like. From the viewpoints of lowcost and ease of handling, phosphoric acid sodium salts are preferable,and sodium dihydrogen phosphate is more preferable.

The concentration of phosphate ions in the pretreatment liquid ispreferably 0.1 to 4.0 mol/L. By setting the concentration of phosphateions to 0.1 mol/L or more, a necessary amount of phosphate ions arecaptured by the anodic oxide film, so that the effect of inhibiting ahydration reaction can be exhibited. In addition, setting theconcentration of phosphate ions to 4.0 mol/L or less makes it possibleto prevent phosphate ions from being excessively captured by the anodicoxide film and allows the reaction for the sealing treatment in the nextstep to proceed appropriately.

Since phosphate ions have the effect of inhibiting a hydration reaction,as described above, excess phosphate ions captured by the anodic oxidefilm may lead to longer sealing treatment time or may stop the sealingreaction. However, it is difficult to completely control the amount ofphosphate ions captured by the anodic oxide film by controlling theconcentration of phosphate ions alone. In this respect, thereaction-controlling agent is added to the pretreatment liquid toprevent phosphate ions from being excessively captured by the anodicoxide film in the present invention.

The reaction-controlling agent is a compound having a carboxy group or asalt thereof, or is a compound capable of forming a hydroxide ion in anaqueous solution. The compound having a carboxy group or the compoundcapable of forming a hydroxide ion in an aqueous solution can preventphosphate ions from being excessively captured by the anodic oxide film,because the carboxy group or the hydroxide ion itself consumes thephosphate ions moderately at a solid-liquid interface between the anodicoxide film and the pretreatment liquid.

A carboxylic acid is preferable as the compound having a carboxy group,because it is acidic and easy to handle. It is possible to use, as thecarboxylic acid, a wide variety of carboxylic acids, for example,saturated carboxylic acids such as formic acid, acetic acid, propionicacid, and butyric acid, dicarboxylic acids such as oxalic acid, malonicacid, succinic acid, glutaric acid, maleic acid, and fumaric acid,aromatic carboxylic acids such as benzoic acid, salicylic acid, phthalicacid, and trimesic acid, hydroxy acids such as citric acid, lactic acid,glycolic acid, malic acid, and tartaric acid, and other carboxylic acidssuch as pyruvic acid, acetoacetic acid, and aconitic acid. In addition,a compound having a relatively low water-solubility such as benzoic acidor salicylic acid may be used in the form of a carboxylic acid salt suchas sodium benzoate or sodium salicylate, instead of the acid form.

As the compound capable of forming a hydroxide ion in an aqueoussolution, it is possible to use a wide variety of compounds includingammonia, carbonates such as sodium hydrogen carbonate, metal saltscapable of forming a hydroxide ion, and the like. Among the compoundscapable of forming a hydroxide ion in an aqueous solution, hydroxidesare preferable, because they are water-soluble and easy to handle.Examples of the hydroxides include sodium hydroxide, calcium hydroxide,magnesium hydroxide, potassium hydroxide, and the like. In particular,sodium hydroxide is preferable, because it is readily available andinexpensive.

The reaction-controlling agent may be a compound having both a carboxygroup and a hydroxy group. Such compounds include the above-describedhydroxy acids such as citric acid, lactic acid, glycolic acid, malicacid, and tartaric acid, and the above-described aromatic hydroxy acidssuch as salicylic acid. Of these compounds, citric acid is preferable,because it is readily available and easy to handle.

The concentration of the reaction-controlling agent in the pretreatmentliquid is preferably at least 0.05 mol/L. By setting the concentrationto 0.05 mol/L or more, it is possible to prevent phosphate ions frombeing excessively captured by the anodic oxide film. An upper limit ofthe concentration of the reaction-controlling agent is not particularlylimited; however, if the concentration of the reaction-controlling agentis excessively high, the viscosity of the pretreatment liquid increases,so that the pretreatment liquid becomes difficult to remove by washingwith water. Hence, the upper limit of the concentration of thereaction-controlling agent is preferably 10 mol/L or less, morepreferably 7 mol/L or less, and further preferably 4 mol/L or less.

The ratio of the concentration of phosphate ions and the concentrationof the reaction-controlling agent is not particularly limited, and, forexample, the ratio of phosphate ion:reaction-controlling agent may be ina range from 1:0.25 to 1:4.

The pH of the pretreatment liquid is in an acidic to neutral range, and,for example, is 1 to 7. This is because, when the pH of the pretreatmentliquid is alkaline, the sealing treatment in the next step is adverselyaffected by the pH, so that the closing of the pores present in ananodic oxide film may be inadequate, or the color tone of the film maychange. In addition, when the pretreatment liquid is in the acidic toneutral range, the following effect is obtained. Specifically, forexample, when a carboxylic acid salt is added to the pretreatmentliquid, the carboxylic acid salt dissociates into a carboxylate anionand a metal ion in the pretreatment liquid. The equilibrium of thecarboxylate anion in the dissociated state moves toward the carboxygroup and exerts an effect of preventing phosphate ions from beingexcessively captured by the anodic oxide film.

In addition, the pH of the pretreatment liquid is preferably in a weaklyacidic to neutral range, for example, from 5 to 7. When the pretreatmentliquid is weakly acidic to neutral as described above, the change in pHof the sealing treatment liquid used in the next step can be reduced,even when the sealing treatment liquid is contaminated with thepretreatment liquid.

For adjusting the pH of the pretreatment liquid to the acidic to neutralrange, the kinds, the concentrations, and the like of the phosphate ionsand the reaction-controlling agent may be adjusted, or an acid such assulfuric acid, nitric acid, hydrochloric acid, or hydrofluoric acid, ora base such as sodium hydrogen carbonate, sodium carbonate, or ammoniamay be used. Note that a silicic acid compound such as sodium silicatecannot be used as the acid or base for adjusting the pH. If used, thesilicic acid compound may inhibit the sealing treatment.

In addition, since the pretreatment liquid contains phosphate ions whichact as a conjugate base, a buffer solution may be prepared by using thephosphate ions in combination with a weak acid or a weak base. By usinga phosphate buffer having a wide-ranging buffer capacity over the weaklyacidic to neutral range with the pH of from 5 to 7, the change in pH ofthe pretreatment liquid can be reduced even when the pretreatment liquidis contaminated with a strongly acidic or strongly alkaline anodictreatment liquid, and the pH of the pretreatment liquid can bemaintained in a predetermined range.

The temperature of the pretreatment liquid is not particularly limited,and it is preferably 10 to 50° C. By setting the temperature to 10° C.or above, the action of the phosphate ions in the pretreatment liquid isactivated, so that the phosphate ions can be appropriately captured bythe anodic oxide film. Meanwhile, by setting the temperature to 50° C.or below, it is possible to prevent a large amount of phosphate ionsfrom being captured by the anodic oxide film, and prevent the change incolor tone of the film due to the dissolution of the anodic oxide filmin the pretreatment liquid and resultant formation of fine projectionsand recesses on the surface. The temperature of the pretreatment liquidis more preferably 15 to 40° C. In this temperature range, thetemperature can be maintained more easily, and the energy necessary formaintaining the temperature can be reduced, because the temperature iscloser to normal temperature.

The immersion time in the pretreatment liquid is preferably at least 1.5minutes. By setting the immersion time to 1.5 minutes or more, phosphateions in the pretreatment liquid can be sufficiently captured by theanodic oxide film. The upper limit of the immersion time is notparticularly limited, and it is preferably 20 minutes or less,considering production efficiency.

After the immersion in the pretreatment liquid, it is preferable toperform washing with water at least once before the sealing treatment inthe next step, as in the case of the step of forming an anodic oxidefilm.

In the sealing treatment step, a sealing treatment using a sealingtreatment liquid containing lithium ions is performed on the anodicoxide film formed on the surface of the aluminum or the aluminum alloy.By this sealing treatment, fine pores present in the surface of theanodic oxide film can be closed. As a source of the lithium ions,lithium sulfate, lithium chloride, lithium nitrate, lithium carbonate,lithium phosphate, lithium hydroxide, a hydrate thereof, or the like canbe used. Of these lithium ion sources, lithium hydroxide and lithiumcarbonate are preferable, because their aqueous solutions are alkalineand nontoxic.

The concentration of lithium ions in the sealing treatment liquid ispreferably 0.02 to 20 g/L, and more preferably 0.08 to 10 g/L. The pH ofthe sealing treatment liquid is preferably at least 10.5, morepreferably at least 11, and further preferably at least 12. An upperlimit of the pH is not particularly limited, and it is preferably atmost 14. Since the pH varies depending on the lithium ion source, the pHof the sealing treatment liquid can be adjusted by using an acid such assulfuric acid, oxalic acid, or chromic acid or a base such as sodiumhydroxide, sodium carbonate, or sodium fluoride.

Regarding a method for the sealing treatment, the sealing treatmentliquid may be attached to the anodic oxide film by immersing thealuminum or the aluminum alloy on which the anodic oxide film is formedin the sealing treatment liquid, spraying the sealing treatment liquidonto the surface of the anodic oxide film, or applying the sealingtreatment liquid onto the surface of the anodic oxide film with an inkbrush or the like. The temperature of the sealing treatment liquid ispreferably 10 to 65° C., and more preferably 25 to 50° C. The time forthe sealing treatment is preferably 0.5 to 5 minutes. After the sealingtreatment liquid is attached, it is preferable to wash the anodic oxidefilm with cold water or hot water and to dry the anodic oxide film byblowing air, using a dryer, or the like.

By such a sealing treatment, a sealing product 26 can be formeduniformly from the film surface to the pore bottom portions in multiplefine pores 24 a present in the surface of an anodic oxide film 22 formedon aluminum or an aluminum alloy 10 as shown in FIG. 1. The sealingproduct 26 may be a lithium-aluminum hydrate 26 a and a boehmite 26 b.The lithium-aluminum hydrate 26 a is, for example, LiH(AlO₂)₂.5H₂O. Theboehmite 26 b is, for example, AlO.OH.

LiH(AlO₂)₂.5H₂O is a pentahydrate, and the formation of thispentahydrate requires 5 water molecules to 1 lithium atom. The phosphateions captured by the anodic oxide film in the pretreatment step have aneffect of inhibiting a hydration reaction and exert a great influence onthe formation of the lithium-aluminum hydrate, which requires a largenumber of water molecules. Hence, the phosphate ions can make theformation reaction of the lithium-aluminum hydrate gentle. Consequently,the conditions (necessary amounts of phosphoric acid and the reactioninhibitor, the immersion time, and the solution temperature) forreducing the change in color tone of the anodic oxide film can bereduced, shortened, and lowered.

In addition, as described above, the sealing product 26 contains theboehmite 26 b in addition to the lithium-aluminum hydrate 26 a. In areference method, a large amount of the lithium-aluminum hydrate 26 a isformed near the surface of the anodic oxide film 22 as shown in FIG. 2,and hence exerts effects on the refraction of light and the like, sothat the film looks white. On the other hand, in the present invention,since the lithium-aluminum hydrate 26 a is uniformly formed in the pores24 of the anodic oxide film as shown in FIG. 1, the change in color toneof the anodic oxide film due to the sealing treatment can beconsequently reduced. In this manner, the present invention makes itpossible to prevent the change in color tone of the anodic oxide film,and to produce an aluminum or aluminum alloy product having a highcorrosion resistance.

EXAMPLES Test Example 1 Regarding Influence of Reaction-ControllingAgent in Pretreatment Liquid

Test pieces made of an aluminum alloy A1100 material were used. This wasbecause of the following reason. Specifically, when a large amount of analloy component is contained, an anodic oxide film is of black or yellowcolor because of the influence of the alloy component. Hence, by usingthe A1100 material which contained almost no alloy component and whichallowed formation of a transparent film, the visual evaluation of theeffect (the reduction of change in color tone) of the present inventionwas facilitated.

In Test Example 1, each test piece of the A1100 material was immersed asan anode in a sulfuric acid bath of a concentration of 200 g/L, and adirect current with a current density of 1.5 A/dm² was applied for 10minutes. Thus, an anodic oxide film with a thickness of 10 μm was formedon a surface of the test piece. Next, for a pretreatment, pretreatmentliquids (20° C.) which were aqueous solutions each having aconcentration of phosphate ions of 0.5 mol/L, a concentration of areaction-controlling agent of 0.25 or 0.5 mol/L, and a pH of 1 to 5 wereprepared by using phosphate ion sources (reagent 1) andreaction-controlling agents (reagent 2) shown in TABLE 1. Note that whenthe aqueous solution was alkaline, the pH was adjusted to the acidicrange by adding sulfuric acid. After the test piece was immersed in eachof the pretreatment liquids for 5 minutes, the pretreated test piece wasimmersed in a sealing treatment liquid (20° C.) which was an aqueoussolution having a concentration of lithium ions of 0.8 g/L and a pH of13 for 1 minute (Examples 1 to 9).

Note that, for comparison, the series of treatments were conducted ontest pieces in the same manner as in the above-described Example, exceptthat pretreatment liquids were prepared by using compounds which did notfall within the reaction-controlling agent of the present invention(Comparative Examples 1 and 2).

Each of the test pieces of Examples and Comparative Examples wasevaluated to determine whether the change in color tone of the anodicoxide film was reduced, and also whether the anodic oxide film wassealed. For the evaluation of the reduction of change in color tone ofthe film, a comparison of color tone with those of test pieces ofReference Examples 1 and 2 described below were made visually.

The test piece of Reference Example 1 was a test piece subjected to onlythe anodic treatment. Specifically, the test piece of Reference Example1 was a test piece on which an anodic oxide film of 10 μm was formed byimmersing a test piece of the A1100 material as an anode in a sulfuricacid bath of a concentration of 200 g/L and applying a direct currentwith a current density of 1.5 A/dm² for 10 minutes.

The test piece of Reference Example 2 was a test piece on which thesealing treatment was performed after the anodic treatment withoutperforming any pretreatment. Specifically, the test piece of ReferenceExample 2 was a test piece on which an anodic oxide film of 10 μm wasformed by immersing a test piece of the A1100 material as an anode in asulfuric acid bath of a concentration of 200 g/L, and applying a directcurrent with a current density of 1.5 A/dm² for 10 minutes, and then onwhich a sealing treatment was performed by immersing the test piece in asealing treatment liquid having a concentration of lithium ions of 0.8g/L and a pH of 13 at a temperature of 20° C. for 1 minute.

Criteria for evaluating whether the change in color tone of the film wasreduced were as follows. Specifically, in a case in which the color ofthe test piece of the Example or Comparative Example was close to thatof the test piece of Reference Example 1 under visual observation, thepretreatment was evaluated to be effective (“good” in the Table),whereas in a case in which the color was close to that of ReferenceExample 2, the pretreatment was evaluated to be ineffective (“inferior”in the Table). FIG. 3 shows the appearance of each of the test pieces ofReference Examples 1 and 2, and, for reference, the appearance of thetest piece (Example 40, described later) for which the pretreatment wasevaluated to be effective.

In addition, for the sealing state, the surface of each anodic oxidefilm was observed under an electron microscope to determine whether theanodic oxide film was sealed. Note that when the sealing product wasformed by the sealing treatment in an amount sufficient to closemultiple fine pores present in the surface of the anodic oxide film, athin plate-shaped substance was observed on the surface of the anodicoxide film as shown in FIG. 4. Accordingly, the sealing state wasevaluated on the basis of the presence or absence of the thinplate-shaped substance in this test. TABLE 1 shows the results of theevaluations described above.

TABLE 1 Concentration of pH of Reduction of phosphate ions Concentrationsolution after change in color Sealing Overall Reagent 1 [mol/L] Reagent2 [mol/L] adjustment tone of film state evaluation Example 1 phosphoricacid 0.5 citric acid 0.25 1 good good good Example 2 disodium hydrogen0.5 citric acid 0.25 3 good good good phosphate Example 3 sodiumdihydrogen 0.5 citric acid 0.25 3 good good good phosphate Example 4trisodium 0.5 citric acid 0.25 3 good good good phosphate Example 5sodium dihydrogen 0.5 oxalic acid 0.25 3 good good good phosphateExample 6 sodium dihydrogen 0.5 tartaric acid 0.5 3 good good goodphosphate Example 7 sodium dihydrogen 0.5 malic acid 0.5 3 good goodgood phosphate Example 8 sodium dihydrogen 0.5 lactic acid 0.5 3 goodgood good phosphate Example 9 sodium dihydrogen 0.5 potassium 0.25 2good good good phosphate hydroxide Comp. Ex. 1 sodium dihydrogen 0.5sulfuric acid 0.5 1 good inferior inferior phosphate Comp. Ex. 2 sodiumdihydrogen 0.5 boric acid 0.5 4 good inferior inferior phosphate

As shown in TABLE 1, in each of Examples 1 to 9 in which thepretreatment liquids were prepared by using various phosphate ionsources and carboxylic acids or various compounds capable of forming ahydroxide ion in an aqueous solution, the sealing was successfullyperformed, while reducing the change in color tone of the anodic oxidefilm. On the other hand, in each of Comparative Examples 1 and 2 inwhich the pretreatment liquids were prepared by using the compoundshaving no carboxy group and being incapable of forming a hydroxide ionin an aqueous solution by itself such as sulfuric acid or boric acid, itwas not possible to control the amount of phosphate ions captured by thefilm, and a large amount of phosphate ions were captured, so that thesealing was performed insufficiently.

Test Example 2 Regarding Influence of pH of Pretreatment Liquid

The series of treatments were conducted on test pieces in the samemanner as in Test Example 1 described above, except that an aqueoussolution (20° C.) was prepared by using phosphoric acid and citric acidwith the concentration of phosphate ions being 0.5 mol/L, and theconcentration of the carboxylic acid being 0.25 mol/L, and that the pHof the aqueous solution was varied in a range from 1 to 13 by addingsodium hydroxide to the aqueous solution in a range from 0 to 3.5 mol/L,to prepare pretreatment liquids (Example 1, Examples 10 to 14, andComparative Examples 3 to 6). Then, the test pieces of these Examplesand Comparative Examples were evaluated for the reduction of change incolor tone of the film and the sealing state in the same manner as inTest Example 1. TABLE 2 shows the results.

TABLE 2 Concentration of Concentration of phosphate ions carboxylic acidConcentration Reduction of [mol/L] [mol/L] of hydroxide Solution changein color phosphoric acid citric acid [mol/L] [pH] tone of film Sealingstate Overall evaluation Example 1 0.5 0.25 pH was adjusted 1 good goodgood Example 10 0.5 0.25 by addition at 0 2 good good good Example 110.5 0.25 to 3.5 mol/L 3 good good good Example 12 0.5 0.25 4 good goodgood Example 13 0.5 0.25 6 good good good Example 14 0.5 0.25 7 goodgood good Comp. Ex. 3 0.5 0.25 8 inferior inferior inferior Comp. Ex. 40.5 0.25 9 inferior inferior inferior Comp. Ex. 5 0.5 0.25 11 inferiorinferior inferior Comp. Ex. 6 0.5 0.25 13 inferior inferior inferior

As shown in TABLE 2, in each of Examples 1 and 10 to 14 in which the pHof the pretreatment liquid was in the acidic to neutral range of from 1to 7, the sealing was successfully performed, while reducing the changein color tone of the anodic oxide film Especially when the pH was 2 to4, the color change reduction effect was high, and more preferredresults were obtained. In each of Comparative Examples 4 to 7 in whichthe pH of the solution was 8 or more, the film surface was dissolved inthe pretreatment liquid, and a lot of fine projections and recesses wereformed, so that the color tone of the film was changed. Hence,Comparative Examples 4 to 7 were not preferable. Moreover, in each ofComparative Examples 3 to 6 in which the pH was 8 or more, even thesealing did not occur. This was presumably because the anodic oxide filmwas also made alkaline by the immersion in the alkaline pretreatmentliquid, and consequently the pH gradient was absent between the film andthe sealing treatment liquid which was also alkaline, so that it becamedifficult for the sealing treatment liquid to penetrate into the film.

Test Example 3 Regarding Influence of Concentration of PretreatmentLiquid

The series of treatments were conducted on test pieces in the samemanner as in Test Example 1 described above, except that pretreatmentliquids were prepared by using sodium dihydrogen phosphate or phosphoricacid with the concentration of phosphate ions being 0 to 5 mol/L or byusing citric acid (a carboxylic acid) or sodium hydroxide (a hydroxide)with the concentration thereof being 0 to 3 mol/L (Example 3, Examples15 to 33, and Comparative Examples 7 to 13). Note that the pHs of thepretreatment liquids were 3 to 4. Then, the test pieces of theseExamples and Comparative Examples were evaluated for the reduction ofchange in color tone of the film and for the sealing state in the samemanner as in Test Example 1. TABLE 3 shows the results.

TABLE 3 Concentration of carboxylic acid Concentration of Reduction of[mol/L] hydroxide [mol/L] Solution change in color Overall citric acidsodium hydroxide [pH] tone of film Sealing state evaluationConcentration of phosphate ions [mol/L] sodium dihydrogen phosphateExample 15 0.1 0.05 0 3 good good good Example 16 0.1 0.25 0 3 good goodgood Example 17 0.25 0.25 0 3 good good good Example 18 0.25 0.5 0 3good good good Example 19 0.5 0.05 0 3 good good good Example 3 0.5 0.250 3 good good good Example 20 0.5 0.5 0 3 good good good Example 21 0.51 0 3 good good good Example 22 1 0.25 0 3 good good good Example 23 10.5 0 3 good good good Example 24 1 1 0 3 good good good Example 25 20.5 0 3 good good good Example 26 2 1 0 3 good good good Example 27 2 20 3 good good good Example 28 3 1 0 3 good good good Example 29 3 2 0 3good good good Example 30 3 3 0 3 good good good Example 31 4 3 1 3 goodgood good Comp. Ex. 7 0.25 0 0 4 good inferior inferior Comp. Ex. 8 0.50 0 4 good inferior inferior Comp. Ex. 9 0.5 0.03 0 4 good inferiorinferior Comp. 0 0.25 0 3 inferior good inferior Ex. 10 Comp. 0.05 0.250 3 inferior good inferior Ex. 11 Comp. 0.05 2 0 3 inferior goodinferior Ex. 12 Comp. 5 2.5 0 3 good inferior inferior Ex. 13 phosphoricacid Example 32 0.5 0 0.5 3 good good good Example 33 1 0 1 3 good goodgood

As shown in TABLE 3, in each of Examples 3 and 15 to 33 in which theconcentration of phosphate ions was 0.1 to 4 mol/L, and theconcentration of the carboxylic acid or the hydroxide was 0.05 to 4mol/L in total, the sealing was successfully performed, while there wasreduction in the change in color tone of the anodic oxide film. On theother hand, in each of Comparative Examples 10 to 12 in which theconcentration of phosphate ions was less than 0.1 mol/L, the amount ofphosphate ions captured by the film was so small that the sealingreaction proceeded rapidly, and consequently, the color of the filmturned white. In addition, in each of Comparative Example 13 in whichthe concentration of phosphate ions exceeded 4 mol/L and ComparativeExamples 7 to 9 in which the concentration of the carboxylic acid or thehydroxide was less than 0.05 mol/L, the sealing did not occur, becausephosphate ions were excessively captured by the film.

Test Example 4 Regarding Influence of Temperature of Pretreatment Liquid

The series of treatments were conducted on test pieces in the samemanner as in Test Example 1 described above, except that a pretreatmentliquid was prepared by using sodium dihydrogen phosphate and citric acidwith the concentration of phosphate ions being 0.5 mol/L, theconcentration of the carboxylic acid being 0.25 mol/L, and the pH being3, and that the test pieces were immersed in the pretreatment liquid atvarious temperatures in the range from 5 to 60° C. (Example 3, Examples34 to 37, and Comparative Examples 14 and 15). Then, the test pieces ofthese Examples and Comparative Examples were evaluated for the reductionof change in color tone of the film and for the sealing state in thesame manner as in Test Example 1. TABLE 4 shows the results.

TABLE 4 Concentration of phosphate ions [mol/L] Concentration ofTemperature of sodium dihydrogen carboxylic acid [mol/L] aqueoussolution Reduction of change Overall phosphate citric acid [° C.] incolor tone of film Sealing state evaluation Example 34 0.5 0.25 10 goodgood good Example 3 0.5 0.25 20 good good good Example 35 0.5 0.25 30good good good Example 36 0.5 0.25 40 good good good Example 37 0.5 0.2550 good good good Comp. Ex. 14 0.5 0.25 5 inferior good inferior Comp.Ex. 15 0.5 0.25 60 inferior inferior inferior

As shown in TABLE 4, in each of Examples 3 and 34 to 37 in which thetemperature of the pretreatment liquid was 10 to 50° C., the sealing wassuccessfully performed, while there was reduction in the change in colortone of the anodic oxide film. On the other hand, in Comparative Example14 in which the temperature was less than 10° C., the action ofphosphate ions in the pretreatment liquid was so sluggish that thephosphate ions were not readily captured by the film. For this reason,it was not possible to inhibit the rapid sealing reaction, and the colortone of the film changed. Meanwhile, in Comparative Example 15 in whichthe temperature exceeded 50° C., the action of phosphate ions wasexcessively activated, and a large amount of phosphate ions werecaptured by the film. Hence, the sealing did not occur. In addition, theanodic oxide film was dissolved in the pretreatment liquid, and a largenumber of fine projections and recesses were formed on the surface, sothat the color tone of the film changed.

Test Example 5 Regarding Influence of Immersion Time in PretreatmentLiquid

The series of treatments were conducted on test pieces in the samemanner as in Test Example 1 described above, except that a pretreatmentliquid was prepared by using sodium dihydrogen phosphate and citric acidwith the concentration of phosphate ions being 0.5 mol/L, theconcentration of the carboxylic acid being 0.25 mol/L, and the pH being3, and that the time for which the test piece was immersed in thepretreatment liquid was varied in the range from 1 to 20 minutes(Example 3, Examples 38 to 43, and Comparative Example 16). Then, thetest pieces of these Examples and Comparative Example were evaluated forthe reduction of change in color tone of the film and for the sealingstate in the same manner as in Test Example 1. TABLE 5 shows theresults.

TABLE 5 Concentration of phosphate ions Concentration of [mol/L]carboxylic acid Reduction of sodium dihydrogen [mol/L] Immersion timechange in color phosphate citric acid [min] tone of film Sealing stateOverall evaluation Example 38 0.5 0.25 1.5 good good good Example 39 0.50.25 2 good good good Example 40 0.5 0.25 3 good good good Example 3 0.50.25 5 good good good Example 41 0.5 0.25 7 good good good Example 420.5 0.25 10 good good good Example 43 0.5 0.25 20 good good good Comp.Ex. 16 0.5 0.25 1 inferior good inferior

As shown in TABLE 5, in each of Examples 3 and 28 to 43 in which theimmersion time in the pretreatment liquid was 1.5 minutes or more, thesealing was successfully performed, while there was reduction in thechange in color tone of the anodic oxide film. On the other hand, inComparative Example 16 in which the immersion time was less than 1.5minutes, the amount of phosphate ions captured by the anodic oxide filmwas so small that it was not possible to inhibit the sealing reaction,and hence the color tone of the film changed.

Test Example 6 Corrosion Resistance Test

Since the aluminum alloy A1100 material, which was used in Test Examples1 to 5, is a corrosion resistant alloy, test pieces of the ADC12material, which is one of the general-purpose aluminum alloys, were usedin Test Example 6 for evaluating corrosion resistance. In Test Example6, each of the test pieces of the ADC12 material was immersed as ananode in a sulfuric acid bath of a concentration of 200 g/L, and adirect current with a current density of 1.5 A/dm² was applied for 10minutes. Thus, an anodic oxide film with a thickness of 4 μm was formedon the surface of the test piece. Next, for the pretreatment, apretreatment liquid (20° C.) which was an aqueous solution having aconcentration of phosphate ions of 0.5 mol/L and a pH of 4 was preparedby using only sodium dihydrogen phosphate, and a pretreatment liquid(20° C.) which was an aqueous solution having a concentration ofphosphate ions of 0.5 mol/L, a concentration of citric acid of 0.25mol/L, and a pH of 3 was prepared by using sodium dihydrogen phosphateand citric acid. After the test pieces were immersed in thesepretreatment liquids for 5 minutes, the pretreated test pieces were eachimmersed in a sealing treatment liquid (20° C.) which was an aqueoussolution having a concentration of lithium ions of 0.8 g/L and a pH of13 for 1 minute.

Then, the test pieces subjected the sealing treatments were subjected toa salt spray test according to JIS Z 2371 for 120 hours and wereevaluated for corrosion resistance on the basis of the presence orabsence of white oxidation. FIG. 5 shows the appearance of the surfacesof the test pieces at this point. In addition, whether the anodic oxidefilm was sealed was also evaluated under an electron microscope.

On the test piece in the case in which the pretreatment liquid containedonly phosphate ions, white oxidation was formed after the corrosionresistance test as shown in FIG. 5( a). This was because phosphate ionswere excessively captured by the film, and the sealing reaction did notoccur. On the other hand, regarding the test piece in the case in whichthe pretreatment liquid contained both phosphate ions and citric acid,the sealing reaction occurred, and the corrosion resistance wasimproved, so that no white oxidation formed, as shown in FIG. 5( b).Note that since a blackish film was formed on the ADC12 material, it wasnot possible to evaluate the change in color tone. In addition, when thefilm surface was observed under an electron microscope, the pores werenot sealed in the case in which the pretreatment liquid contained onlyphosphate ions. In the case in which the pretreatment liquid containedboth phosphate ions and citric acid, the pores were confirmed to besealed. These results were in agreement with the results of the actualcorrosion resistance test.

What is claimed is:
 1. A method for forming a film on a surface ofaluminum or an aluminum alloy, comprising the steps of: forming ananodic oxide film on a surface of aluminum or an aluminum alloy;performing a pretreatment by immersing, in a pretreatment liquid, thealuminum or the aluminum alloy on which the anodic oxide film is formed;and performing a sealing treatment using a sealing treatment liquidcontaining lithium ions on the anodic oxide film present on the surfaceof the aluminum or the aluminum alloy subjected to the pretreatment,wherein the pretreatment liquid contains phosphate ions and areaction-controlling agent and has a pH in a neutral to acidic range,and the reaction-controlling agent contains a compound having a carboxygroup or a salt thereof or contains a compound capable of forming ahydroxide ion in an aqueous solution.
 2. The method according to claim1, wherein a concentration of the phosphate ions is 0.1 to 4.0 mol/L. 3.The method according to claim 1, wherein a concentration of thereaction-controlling agent is at least 0.05 mol/L.
 4. The methodaccording to claim 1, wherein a temperature of the pretreatment liquidis 10 to 50° C.
 5. The method according to claim 1, wherein an immersiontime in the pretreatment liquid is at least 1.5 minutes.
 6. The methodaccording to claim 1, wherein the compound having a carboxy group is acarboxylic acid or a salt thereof.
 7. The method according to claim 1,wherein the compound capable of forming a hydroxide ion in an aqueoussolution is a hydroxide.
 8. The method according to claim 1, wherein theanodic oxide film has a thickness of 3 to 40 μm.
 9. The method accordingto claim 1, wherein in the sealing treatment step, a sealing product isformed in pores present in a surface of the anodic oxide film, and thesealing product is a lithium-aluminum hydrate containing at leastLiH(AlO₂)₂.5H₂O.
 10. The method according to claim 1, wherein thepretreatment liquid is a buffer solution.
 11. A pretreatment liquid usedfor a pretreatment in a sealing treatment on an anodic oxide film byusing a sealing treatment liquid containing lithium ions, thepretreatment liquid comprising: phosphate ions; and areaction-controlling agent, wherein the pretreatment liquid has a pH ina neutral to acidic range, and the reaction-controlling agent contains acompound having a carboxy group or a salt thereof or contains a compoundcapable of forming a hydroxide ion in an aqueous solution.
 12. Analuminum or aluminum alloy product comprising: aluminum or an aluminumalloy; and an anodic oxide film formed on a surface of the aluminum orthe aluminum alloy, wherein pores in a surface of the anodic oxide filmformed on the aluminum or the aluminum alloy are sealed with a sealingproduct, the sealing product contains a lithium-aluminum hydrate, andthe lithium-aluminum hydrate is uniformly formed from the surface of theanodic oxide film to pore bottom portions in the pores.